Environmental pollution is a serious problem which is especially acute in urban areas. A major cause of this pollution is exhaust emissions from automotive vehicles. Official standards have been set for regulating the allowable amounts of pollutants species in automobile exhausts, and in some areas, periodic inspections or "smog checks" are required to ensure that vehicles meet these standards.
However, there are still large numbers of vehicles operating on public highways which fail to comply with the standards. It has also been determined that a disproportionately large amount of pollution is generated by a relatively small number of vehicles.
Highly polluting vehicles can operate even in areas in which periodic emission inspections are required. Some older vehicles and special types of vehicles are exempt from inspections.
Anti-pollution devices which are required equipment on newer vehicles accomplish their intended purpose of reducing pollution in the vehicle exhaust to within prescribed levels. However, it is perceived by some vehicle owners that antipollution equipment reduces engine performance.
For this reason, some vehicle owners with mechanical expertise can perform whatever servicing is necessary to place their vehicles in condition to pass required inspections, and subsequently remove anti-pollution devices and/or return the vehicles with an attendant increase in pollutant emissions for normal use.
An anti-pollution program which depends entirely on mandatory periodic inspections performed at fixed facilities is therefore inadequate. It is necessary to identify vehicles which are actually operating in violation of prescribed emission standards, and either require them to be placed in conformance with the standards or be removed from operation.
A system for remote sensing of automotive exhaust emissions is described in an article entitled "ANALYTICAL APPROACH--IR Long-Path Photometry: A Remote Sensing Tool for Automotive Emissions:, by G. Bishop et al., in Analytical Chemistry 1989, 61, 617A. An infrared beam is transmitted through the exhaust plume of an automotive vehicle to a sensor unit which includes a beam splitter which splits the beam into a carbon dioxide (CO.sub.2) channel and a carbon monoxide (CO) channel.
The beam in the CO.sub.2 channel passes through a bandpass filter which isolates the spectral absorption region of carbon dioxide and is incident on a photovoltaic detector. The beam in the CO channel passes through a rotating gas filter wheel, one-half of which contains a CO and hydrogen (H.sub.2) mixture, and the other half of which contains nitrogen (N.sub.2). From the filter wheel, the beam in the CO channel passes through another bandpass filter which isolates the spectral absorption region of carbon monoxide and is incident on another photovoltaic detector.
The output signals of the detectors vary in accordance with the transmittance of the vehicle exhaust plume at the respective wavelengths, and thereby the concentrations of CO and CO.sub.2 in the plume. The CO/H.sub.2 portion of the filter wheel provides a reference output, whereas the N.sub.2 portion provides a carbon monoxide output.
Baseline sensor outputs are obtained with no vehicle passing through the beam, and with the beam blocked by a vehicle prior to sensing of the plume. These values are used as references for calibrating the outputs of the detectors when the plume is actually sensed. The detector outputs, which correspond to the transmittances at the respective wavelengths, are then processed in accordance with predetermined functions to determine the relative percentages of CO.sub.2 and CO in the vehicle exhaust plume.
This system is said to be capable of sensing the exhaust gas composition of moving vehicles, and to be useful in identify polluting vehicles for enforcement purposes. However, it suffers from certain drawbacks.
For example, precise alignment is required to ensure that the beams in the two paths are incident on the detectors in an identical manner. A small misalignment error can seriously degrade the measurement accuracy. The two photovoltaic detectors are remote from each other, and require separate cooling units for temperature regulation. A small difference in temperature, as well as small mismatches in other characteristics of the detectors, can also seriously degrade the measurement accuracy.
The rotating filter wheel is a mechanical unit which is expensive and prone to mechanical malfunction. The concentrations of the gases in the filter must be maintained at precise values in order to obtain accurate measurements. The system is also difficult to expand for sensing of additional pollutant species, since each new channel will require another beam splitter, detector, etc. and involve the problems described above.
One particularly noxious pollutant that is not sensed at all by known types of systems, or that is only inaccurately sensed, is nitric oxide (NO). Standard infrared techniques such as FTIR fail in quantifying NO in the atmosphere because of the significant interference resulting from water absorption in absorbing bands in the region around 5.2 .mu.m and 6.2 .mu.m in which NO absorbs. Attempts to subtract the water absorption band are not successful due to the limited accuracy with which water absorption signature can be modeled over the entire spectral region in which FTIR, by its nature, must scan.
Alternate IR laser approaches that use overtone techniques are also not adequate, due at least in part to the requirement to compensate for the water vapor absorption, and to a requirement to provide very accurate temperature control.
An alternative approach utilizes UV absorption in the spectral region around 270 nm. Although NO absorption is strong in this spectral region, the application of this approach to moving vehicles is difficult due to interference from natural pollutants present in the automotive exhaust, i.e., the aromatics Benzene and Toluene. The multiplicity of aromatics emitted in a typical exhaust plume, and the absorption caused thereby, makes compensation for the aromatics very difficult, and also limits the accuracy of the measurement.
It should be appreciated that many of these problems are compounded when the exhaust gas pollution detection system is required to be portable, and also to be capable of being operated in less than ideal surroundings, such as when it is desired to monitor vehicles that are traveling on a roadway, such as a highway or freeway.